As a technique for reducing surface reflection, there is conventionally known a light interference film configured by stacking layers having different refractive indices. In recent years, studies related to an uneven structure having a smaller size than a visible light wavelength, that is, a so-called Moth-eye structure have been advanced as a technique capable of remarkably reducing surface reflection as compared with the light interference film.
The Moth-eye structure is an uneven structure which is much finer than an uneven structure formed on an antiglare (AG) film and has a smaller size than a visible light wavelength. According to the Moth-eye structure, a change in a refractive index can be artificially continuous in a boundary between an outside (air) and an article which are media having different refractive indices from each other. For this reason, it is possible to suppress reflection of light generally occurring in a boundary between the media having the different refractive indices from each other. By disposing the Moth-eye structure on a surface of an article which is to be subjected to antireflection processing, accordingly, it is possible to considerably suppress the reflection of the light on the surface of the article, thereby enhancing a transmissivity of light remarkably.
The Moth-eye structure is extremely effective as means for enhancing visibility of a display device. When the display device is to be utilized in a bright place, particularly, a large quantity of outside light is incident on an outermost surface of the display device. For this reason, a surface reflectance should be reduced sufficiently. Otherwise, a ratio of a quantity of reflected light to a quantity of displaying light emitted from an inner part of the display device is excessively increased so that clear display cannot be obtained. By utilizing the Moth-eye structure, it is possible to sufficiently reduce the surface reflectance on the outermost surface of the display device. Therefore, it is possible to prevent decrease in a contrast ratio of display images in the bright place.
Examples of a method of disposing the Moth-eye structure on the outermost surface of the display device include a method of attaching an antireflection film including the Moth-eye structure on a surface to a surface of an article which is to be subjected to antireflection processing. As a method of manufacturing the antireflection film including the Moth-eye structure on the surface, there is known a method of pushing a mold including, on a surface, a structure obtained by inverting the Moth-eye structure against a surface of a base film and transferring the inverted structure formed on the surface of the mold onto the surface of the base film. As a method of forming the inverted structure on the surface of the mold, moreover, there is known a method of anodizing a surface of a mold to form an oxide film, thereby etching the oxide film selectively.
The Moth-eye structure is disposed on the outermost surface of the display device in many cases. In recent years, many display devices include touch panels. For this reason, the Moth-eye structure is required to also have characteristics, for example, a mechanical strength and a contamination resistance. On the other hand, it is considered to provide a covering layer on the surface of the Moth-eye structure. For example, Patent Literature 1 describes a covering layer consisting of a hydrolytic condensation product of a silane coupling agent. Patent Literature 2 describes a transparent thin film such as silicon dioxide and a layer which has a film thickness of several nm or less and is excellent in an oil repelling property (see paragraph 0045). Patent Literature 3 describes a film having low surface energy such as a water repellent coating film made of polytetrafluoroethylene in a film thickness of 100 Å to 10000 Å (see claims 2 and 3, and paragraphs 0036 and 0037). Patent Literature 4 describes an antifouling function layer consisting of a material containing fluorine and formed with an uneven structure maintained (see claim 8, paragraph 0114). Patent Literature 5 describes resin coating having a greater contact angle with water than 90° and the resin coating having a smaller contact angle with water than 90° (see claims 6 and 7). Patent Literature 6 describes a functional layer formed by directly chemical bond of a compound containing silicon to a surface of a fine protrusion. Patent Literature 7 describes a transparent conductive film taking a shape conforming to a shape of a structure and having a film thickness of 9 to 50 nm (see claim 1, and paragraphs 0020 and 0021). Patent Literature 8 describes a transparent conductive film having the greatest average film thickness in a top part of a structure (see claim 5). Patent Literature 9 describes a transparent conductive thin film and an opaque thin film (see claims 1 and 6).
However, a conventional coating layer provided on the surface of the Moth-eye structure is formed in a uniform thickness or in an extremely small film thickness on the surface of the Moth-eye structure so as not to vary a reflectance characteristic of the Moth-eye structure.